We have shown that the receptor mechanisms of isolated, organ-cultured SON of puppies confirms to the properties of these same neurones in situ. Therefore, we are encouraged to believe that this may become an ideal in vitro system for studying regulation of neurosecretion. Projected Studies: a. Fine structure analysis of the SON cells in culture, to determine under what conditions the most typical neurosecretory axonal outgrowth can be observed. b. Begin the study of vasopressin synthesis of the explanted SON, both by autoradiography in the electron microscope and by isolation of labeled vasopressin.